The installation, to be built in the shape of a grotto next to the Cathedral Basilica of Ss. Peter and Paul in Logan Square, is inspired by a painting called "Mary, Undoer of Knots" that holds special meaning for the pope.

People will be invited to write down their own problems on provided material, most likely ribbon, and tie them to the grotto, leaving them behind. They'll also be encouraged to help others by loosening and removing a knot already in place.

"It's a grotto made of struggles, where people can come and leave their problems and perhaps carry someone's load for a little bit," said artist Meg Saligman. "This is just the beginning . . . We keep playing with it."

Saligman is still unsure what shape the grotto will take or what material she will use to construct it. But she hopes to gather hundreds of thousands of knots before the September papal visit by holding workshops throughout the city and allowing others to send in their problems via the website mercyandjustice.org.

Some knots already collected address specific issues, like "I pray for the strength to survive these medical issues." Others are more general, like "I have trouble finding balance, peace, in my life."

One writer asks that the pope pray for Betty Boop, a dog.

Francis first encountered the 18th-century painting "Mary, Undoer of Knots" as a young man. The piece shows Mary surrounded by angels and trying to loosen one of the knots along a ribbon that stretches to the ground.

In 1998, when Francis was named archbishop of Buenos Aires, he encouraged people to pray to Mary.

"I entrust into your hands the ribbon of my life," the prayer says. "In your hands, there is no knot that cannot be undone."

The pope's first U.S. tour was spurred by Philadelphia's hosting the 2015 World Meeting of Families on Sept. 22 through 25, a triennial event organized by the Pontifical Council for Families.

Francis is scheduled to speak at one of the meeting's closing events Sept. 26 and to lead a public Mass in front of the Philadelphia Museum of Art the next day. Event planners expect more than a million people to gather along Benjamin Franklin Parkway to take part in that service.

Before Philadelphia, Francis will visit the White House, address Congress and speak at the United Nations in New York.

The grotto installation is being financed by the nonprofit homeless advocacy group Project Home. Sister Mary Scullion, the organization's founder and chair of the World Meeting's Hunger and Homelessness Committee, said the project will serve as a reminder that everyone has struggles, even if they can't be seen.

"We're hoping that people are moved to acknowledge a higher power and to also acknowledge the power within ourselves to act," Scullion said. "We need God's grace to untie the knots, but we also need each other."